The yeast RAD52 epistasis group of genes are involved in DNA recombination and repair of DNA damaged by radiation and other environmental agents such as cross-linking and alkylating agents. While screening for proteins that associate with human RAD52 protein using the yeast two hybrid system, we have cloned the human homolog of yeast ubiquitin-conjugating enzymes S. cerevisiae UBC9 or S. pombe Hus5. Human UBC9 also associates with RAD51, tumor suppressor protein p53 and a newly identified mitosis-related protein hRAP12. Considering these interactions and the fact that yeast UBC9 homologs are involved in S- and M-phase cyclin degradation, mitosis control and radiation sensitivity, we suggest that human UBC9 participates in repairing DNA damage induced by environmental agents, cell cycle control, and p53- mediated processes. The objective of this research is to understand the functions and mechanisms of UBC9 in the context of DNA repair, cell cycle control, and p53-mediated processes after DNA has been damaged by radiation and other environmental stresses. The specific aims for this FIRST proposal are: 1)to determine whether human UBC9 protein forms a stable complex with RAD51, RAD52, p53, and hRAP12; 2)to determine whether direct protein-protein binding is involved in these interactions; 3)to characterize the regulation of human UBC9 expression by radiation exposure and during the cell cycle; 4)to determine whether increased/reduced expression of human UBC9 protein will affect cellular resistance to radiation and cell cycle progression; 5)to identify novel proteins that may associate with UBC9, RAD51, RAD52, and hRAP12. It is expected that once the proposed goals are accomplished, we will gain more insights about how environmentally induced DNA damage is repaired, and how the repair processes are coordinated with other processes, such as cell cycle control and p53-mediated processes.